Commit message: io_uring: reexpand under-reexpanded iters

[   74.211232] BUG: KASAN: stack-out-of-bounds in iov_iter_revert+0x809/0x900
[   74.212778] Read of size 8 at addr ffff888025dc78b8 by task
syz-executor.0/828
[   74.214756] CPU: 0 PID: 828 Comm: syz-executor.0 Not tainted
5.14.0-rc3-next-20210730 #1
[   74.216525] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[   74.219033] Call Trace:
[   74.219683]  dump_stack_lvl+0x8b/0xb3
[   74.220706]  print_address_description.constprop.0+0x1f/0x140
[   74.224226]  kasan_report.cold+0x7f/0x11b
[   74.226085]  iov_iter_revert+0x809/0x900
[   74.227960]  io_write+0x57d/0xe40
[   74.232647]  io_issue_sqe+0x4da/0x6a80
[   74.242578]  __io_queue_sqe+0x1ac/0xe60
[   74.245358]  io_submit_sqes+0x3f6e/0x76a0
[   74.248207]  __do_sys_io_uring_enter+0x90c/0x1a20
[   74.257167]  do_syscall_64+0x3b/0x90
[   74.257984]  entry_SYSCALL_64_after_hwframe+0x44/0xae

old_size = iov_iter_count();
...
iov_iter_revert(old_size - iov_iter_count());

If iov_iter_revert() is done base on the initial size as above, and the
iter is truncated and not reexpanded in the middle, it miscalculates
borders causing problems. This trace is due to no one reexpanding after
generic_write_checks().

Now iters store how many bytes has been truncated, so reexpand them to
the initial state right before reverting.

Cc: stable@vger.kernel.org
Reported-by: Palash Oswal <oswalpalash@gmail.com>
Reported-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Reported-and-tested-by: syzbot+9671693590ef5aad8953@syzkaller.appspotmail.com
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
CVE: CVE-2022-1508
CWE: CWE-125
Vulnerability description: An out-of-bounds read flaw was found in the Linux kernel’s io_uring module in the way a user triggers the io_read() function with some special parameters. This flaw allows a local user to read some memory out of bounds.

Project: linux
File: io_uring.c

static int io_read(struct io_kiocb *req, unsigned int issue_flags)
{
	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
	struct kiocb *kiocb = &req->rw.kiocb;
	struct iov_iter __iter, *iter = &__iter;
	struct io_async_rw *rw = req->async_data;
	ssize_t io_size, ret, ret2;
	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;

	if (rw) {
		iter = &rw->iter;
		iovec = NULL;
	} else {
		ret = io_import_iovec(READ, req, &iovec, iter, !force_nonblock);
		if (ret < 0)
			return ret;
	}
	io_size = iov_iter_count(iter);
	req->result = io_size;

	/* Ensure we clear previously set non-block flag */
	if (!force_nonblock)
		kiocb->ki_flags &= ~IOCB_NOWAIT;
	else
		kiocb->ki_flags |= IOCB_NOWAIT;

	/* If the file doesn't support async, just async punt */
	if (force_nonblock && !io_file_supports_async(req, READ)) {
		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, true);
		return ret ?: -EAGAIN;
	}

	ret = rw_verify_area(READ, req->file, io_kiocb_ppos(kiocb), io_size);
	if (unlikely(ret)) {
		kfree(iovec);
		return ret;
	}

	ret = io_iter_do_read(req, iter);

	if (ret == -EAGAIN || (req->flags & REQ_F_REISSUE)) {
		req->flags &= ~REQ_F_REISSUE;
		/* IOPOLL retry should happen for io-wq threads */
		if (!force_nonblock && !(req->ctx->flags & IORING_SETUP_IOPOLL))
			goto done;
		/* no retry on NONBLOCK nor RWF_NOWAIT */
		if (req->flags & REQ_F_NOWAIT)
			goto done;
		/* some cases will consume bytes even on error returns */
		iov_iter_revert(iter, io_size - iov_iter_count(iter));
		ret = 0;
	} else if (ret == -EIOCBQUEUED) {
		goto out_free;
	} else if (ret <= 0 || ret == io_size || !force_nonblock ||
		   (req->flags & REQ_F_NOWAIT) || !(req->flags & REQ_F_ISREG)) {
		/* read all, failed, already did sync or don't want to retry */
		goto done;
	}

	ret2 = io_setup_async_rw(req, iovec, inline_vecs, iter, true);
	if (ret2)
		return ret2;

	iovec = NULL;
	rw = req->async_data;
	/* now use our persistent iterator, if we aren't already */
	iter = &rw->iter;

	do {
		io_size -= ret;
		rw->bytes_done += ret;
		/* if we can retry, do so with the callbacks armed */
		if (!io_rw_should_retry(req)) {
			kiocb->ki_flags &= ~IOCB_WAITQ;
			return -EAGAIN;
		}

		/*
		 * Now retry read with the IOCB_WAITQ parts set in the iocb. If
		 * we get -EIOCBQUEUED, then we'll get a notification when the
		 * desired page gets unlocked. We can also get a partial read
		 * here, and if we do, then just retry at the new offset.
		 */
		ret = io_iter_do_read(req, iter);
		if (ret == -EIOCBQUEUED)
			return 0;
		/* we got some bytes, but not all. retry. */
		kiocb->ki_flags &= ~IOCB_WAITQ;
	} while (ret > 0 && ret < io_size);
done:
	kiocb_done(kiocb, ret, issue_flags);
out_free:
	/* it's faster to check here then delegate to kfree */
	if (iovec)
		kfree(iovec);
	return 0;
}